An optical pickup is used for recording and/or playing information to/in an optical disc such as CD and DVD. The optical pickup includes a quarter-wave plate in order to convert a linearly polarized laser beam irradiated onto the optical disc to a circularly polarized laser beam. The quarter-wave plate is an optical element performing a phase modulation by using a material having birefringence and has a function to convert an incoming light beam of linearly polarized light into a light beam of circularly polarized light. On the other hand, since a retardation of the wave plate is a function of a wavelength, the wave plate has a wavelength dependence in which if a wavelength to be used is changed, the retardation is also changed. Therefore, when a plurality of laser beams having a different wavelength enter the wave plate, a retardation varies depending on each of the wavelengths, causing an issue in which the retardation of the wave plate cannot be maintained constant.
Then, to solve this problem, JP-A-10-68816 discloses a technology that broadens a band of a quarter-wave plate.
FIG. 7 shows an external view of a broadband quarter-wave plate in related art disclosed in JP-A-10-68816. FIG. 7(a) is a plan view of the wave plate as seen from an incident direction whereas the FIG. 7(b) shows a side view. This broadband quarter-wave plate is provided with two wave plates having a predetermined retardation that are laminated. A half-wave plate and a quarter-wave plate are laminated so that respective optical axes are intersected at a predetermined angle, providing performance as a desired quarter-wave plate. As shown in FIG. 7, a laminated wave plate 101 serves as a broadband quarter-wave plate and includes a first wave plate 102 and a second wave plate 103 laminated each other so that an optic axis orientation (hereinafter referred to as an in-plane azimuth) θ1 of the first wave plate 102 is 15 degrees and an in-plane azimuth θ2 of the second wave plate 103 is 75 degrees.
Next, a design method for the broadband quarter-wave plate according to the related art will be explained.
The two wave plates composing the broadband quarter-wave plate 101 will be explained below using a Poincare sphere showing a polarization state in which a light beam is entered into the broadband quarter-wave plate 101 when a retardation Γ1 of the first wave plate 102 is 180 degrees and an in-plane azimuth thereof is θ1 whereas a retardation rΓ2 of the second wave plate 103 is 90 degrees and an in-plane azimuth thereof is θ2. Note that each of the retardation Γ1 (=180 degrees) of the first wave plate 102 and the retardation Γ2 (=90 degrees) of the second wave plate 103 is a retardation with respect to a reference wavelength (design wavelength) λ0.
FIG. 8 shows a Poincare sphere showing a polarization state in which a light beam is entered into the broadband quarter-wave plate 101 in related art. Here, the broadband quarter-wave plate 101 is composed of the half-wave plate 102 having an optical axis that is θ1 and the quarter-wave plate 103 having an optical axis that is θ2. However, on the Poincare sphere, the optical axis of the half-wave plate 102 is represented as a straight line R1, whereas the optical axis of the quarter-wave plate 103 is represented as a straight line R2.
FIG. 8 shows that when a light beam enters as linearly polarized light 101 that is in parallel to the equator from a predetermined position P0 on the equator of the Poincare sphere. First, in a case of the reference wavelength λ0, the light beam is rotated by 180 degrees around the optical axis R1 as a center by the half-wave plate 102 to be shifted to P1, further, rotated by 90 degrees around the optical lens R2 as a center by the quarter-wave plate 103 to reach P2 (North Pole), so that the light beam outputs from the laminated wave plate 101 turns to be circularly polarized light. To make P2 be the pole of the Poincare sphere here, θ1 and θ2 should satisfy a following formula.θ2=2θ1+45  (101)
As described above, when a wavelength of the incident light changes between the wavelengths λ1 and λ2 (λ1<λ0<λ2) from the reference wavelength (designed wavelength) λ0, the retardations of the first wave plate 102 and the second wave plate 103 change from 180 degrees and 90 degrees respectively, due to wavelength dependences of the wave plates. At this time, a change amount of the first wave plate 102 is ΔΓ1, and a change amount of the second wave plate 103 is ΔΓ2. Here, if P1 on the Poincare sphere, which is a position where the incident light reaches by being modulated from P0 by the first wave plate 102, is shifted to P1′ due to a change of the retardation of the wave plate along with a change of the wavelength of the incident light, P2 can always reach the polar of the Poincare sphere under a condition where the change amounts ΔΓ1 and ΔΓ2 are on the same circle connecting P1 and P1′ along the surface of the Poincare sphere.
Therefore, when P1 and P1′ are approximately connected by a straight line, a relation among ΔΓ1, ΔΓ2 and θ1 is shown by a cosine theorem as below.cos ΔΓ2=1−2(1−2 cos 2θ1)(1−cos ΔΓ1)  (102)
If the first wave plate 102 and the second wave plate 103 are the same factors for chromatic dispersion, respective retardations are 180 degrees and 90 degrees. Therefore, a relation between ΔΓ1 and ΔΓ2 satisfies a following formula.ΔΓ1=2ΔΓ2  (103)
When the formula (103) is substituted into the formula (102), θ1 obtained is:                θ1=15° (approximately).        
Further, θ2 obtained from the formula (101) is:                θ2=75° (approximately.        
Accordingly, arranging θ1 and θ2 as described above enables the wavelength dependence of the first wave plate 102 and the wavelength dependence of the second wave plate 103 to be cancelled out each other.
According to the result above, following conditions are essential in order to make the laminated wave plate 101 perform as a broadband quarter-wave plate.
The first wave plate 102Retardation Γ1180degreesIn-plane azimuth θ115degreesThe second wave plate 103Retardation Γ290degreesIn-plane azimuth θ275degrees
Note that the conditions above include approximations. Therefore, optimization is performed by simulations with Mueller matrix and Jones vector or the like so as to obtain optimum characteristics for a practical use.
Patent Document 1 JP-A-10-68816
Patent Document 2 JP-A-10-340471